The PEFC includes a layer of a membrane-electrode assembly (MEA) and a separator. The MEA includes an electrolyte membrane of an ion-exchange membrane and a pair of electrodes disposed on opposite sides of the membrane. The pair of electrodes includes an anode disposed on one side of the membrane and including a first catalyst layer and a cathode disposed on the other side of the membrane and including a second catalyst layer. The separator includes a fuel gas passage formed therein for supplying fuel gas (e.g., hydrogen) to the anode, or an oxidant gas passage for supplying oxidant gas (e.g., oxygen, usually, air) to the cathode, and/or a coolant passage formed therein for letting coolant (usually, cooling water) flow through the coolant passage. A gas diffusion layer may be disposed between the MEA and the separator on an anode side and a cathode side of the MEA.
On the anode side of each cell, there occurs a reaction that hydrogen changes to hydrogen ions (i.e., protons) and electrons. The hydrogen ions move through the electrolyte membrane to the cathode where the hydrogen ions react with oxygen supplied and electrons (which are generated at an anode of the adjacent MEA and move to the cathode of the instant MEA through a separator, or which are generated at an anode of a fuel cell located at one end of a fuel cell stack and move to a cathode of a fuel cell located at the other end of the fuel cell stack through an external electrical circuit) to form water as follows:                At the anode: H2→2H++2e−        At the cathode: 2H++2e−+(½)O2→H2O        
The product water increases in amount in a downstream portion of the oxidant gas passage and is likely to cause flooding. In the flooding region, supply of the oxidant gas to cathode is obstructed, and the above-described reaction does not occur smoothly. As a result, a power generation ability drops. Hence, removal of product water thereby suppressing flooding is important.
A similar problem occurs with the fuel gas passage, since a portion of the water in the oxidant gas passage move through the electrolyte membrane into the fuel gas passage. In order that the above-described reaction is conducted smoothly, the electrolyte membrane has to be properly in a wet state and the oxidant gas and the fuel gas are humidified before being supplied to the fuel cell. Therefore, flooding is more likely to occur.
Japanese Patent Publication No. HEI 11-508726 discloses that an entire portion of a separator is made from porous material and water produced at the cathode is pushed through a porous separator into the cooling water passage so that flooding is prevented.
However, with the fuel cell in which the water produced on the cathode is pushed into the cooling water passage as is disclosed in Japanese Patent Publication No. HEI 11-508726, there are the following problems:
In a case where the cooling water is anti-freeze coolant, some components in the coolant may damage the electrolyte membrane when the coolant moves to the oxidant gas passage. For this reason, pure water is used in the fuel cell of Japanese Patent Publication No. HEI 11-508726, so that operation of the fuel cell at temperatures below the freezing point is impossible. Further, impurities in the oxidant gas passage permeate the separator into the cooling water so that an ion-conductivity of the cooling water increases.
Further, in order to cause the product water to move to/into the cooling water passage, control of a pressure difference between the gas passage and the coolant passage is required, and a system therefor is complex.
Further, since the product water is pushed out into the cooling water, the product water cannot be utilized for humidifying the reactant gas.
Furthermore, uniform pushing out of product water is impossible, since the oxidant passage includes ribs and grooves and a passing-through characteristic of product water is different between the rib portion and the groove portion of the separator.